Cation non-stoichiometry in multi-component oxidenanoparticles by solution chemistry: a case study on CaWO4 for tailored structural properties

Abstract

Chemical composition directly determines the structure and properties of almost all bulk inorganic solids, which are however popularly dismissed in the literature as a cause of property changes when studying multi-component oxide nanostructures by solution chemistries. The current work focuses on this subject through a systematic case study on CaWO₄ nanocrystals. CaWO₄ nanocrystals were prepared using room-temperature solution chemistry, in which a capping agent of citric acid was employed for kinetic grain size control. Sample characterizations by a set of techniques indicated that 5–7 nm CaWO₄ was obtained at room temperature, showing a pure-phase of tetrahedral scheelite structure. The molar ratio of Ca²⁺ to W⁶⁺ was found to be 1.2 : 1, apparently deviating from the unity expected for the stoichiometric CaWO₄. Such nonstoichiometry was further modulated via iso-valent incorporation of smaller Zn²⁺ to the Ca²⁺-sites in CaWO₄. It is found that with increasing the Zn²⁺ content, there appeared transformation from high to low nonstoichiometry, though a pure scheelite-typed structure was retained. Such a nonstoichiometry was primarily represented by excessive cations like Zn²⁺ and/or Ca²⁺ within the surface disorder layers, which in turn showed a great impact on the structure and properties as demonstrated by a lattice contraction, band-gap narrowing, luminescence quenching, as well as improved conductivity. The property changes were rationalized in terms of surface structural disorder, electro-negativity discrepancy, and effective activation on the mobile protons. Consequently, systematic control over the non-stoichiometry for single-phase multi-component oxide nanostructures by solution chemistry is proven fundamentally important, which may help to achieve quantitatively the structure–property relationship for materials design and performance optimization.